The present invention relates to the sterility assurance arts. It finds particular application in connection with the distribution of microbial spores onto a spore carrier, and will be described with particular reference thereto.
Currently, the instrument of choice for monitoring the effectiveness of a sterilization process is the biological indicator. A typical biological indicator contains a calibrated population of microorganisms having a high resistance to the sterilization process under investigation. After exposure to the sterilization process, the biological indicators are incubated in a culture medium to encourage growth of any remaining viable microorganisms. Self-contained biological indicators contain the culture medium within the indicator, typically in a frangible vial. Spore strip biological indicators are combined with a separate container of culture medium after the monitored sterilization process. Subsequent microbial growth is an indication that the sterilization process was ineffective.
Bacterial spores are favored for biological indicator microorganisms because they are typical of the worst case microbes that are targeted in the sterilization procedure. The selected spores are highly resistant to the physical and chemical agents utilized in the sterilization process. The biological stability of the spores permits the manufacture of a product that exhibits a long shelf life relative to one comprising vegetative cells. The choice of bacteria is dependent on the sterilization to be evaluated. For example, Bacillus stearothermophilus spores are used to monitor moist heat sterilization and hydrogen peroxide sterilization because of their high resistance to these processes. Similarly, Bacillus subtilis spores are employed to monitor ethylene oxide sterilization, dry heat sterilization, and sterilization systems utilizing peroxy compounds in the plasma state. Because the type of microorganisms in the biological indicator are not readily identifiable by the user, the culture medium of self-contained biological indicators is sometimes color-coded according to the type of sterilization process to be evaluated.
The microorganisms are generally supported on a carrier, such as a spore strip or disk. The carrier is formed from a material which is resistant to the sterilization process and does not contain additives which may influence the sterility assessment. Materials, such as filter paper, chromatography paper, blotter paper, glass fibers, polymer plastics, and stainless steel articles are among those most often used for the carrier.
To distribute the microorganisms on the carrier, a suspension of microorganisms in water is conventionally pumped to a needle which is suspended over a web of the paper or other carrier material. The paper is moved under the needle at a constant rate, causing a trail of the suspension to form on the paper as it passes beneath the needle. Alternatively, the suspension is manually transferred by use of a micropipette to the carrier. The web of impregnated paper is then cut to the appropriate size for inserting into the indicator, typically as strips or disks.
It is important that each of the spore strips or disks contains a calibrated population of the microorganisms. The rate of delivery of the suspension and the rate of transport of the paper are adjusted so that the web receives an even distribution of a selected concentration of the microorganisms. However, occasional malfunctioning of the delivery system can result in uneven distribution of the microorganisms. For example, the needle delivering the microorganism suspension sometimes becomes blocked and the suspension is not delivered to the paper web. Additionally, the pumps used to deliver the microorganism suspension to the needles are generally peristaltic pumps which deliver the suspension as a series of short pulses. This can cause the suspension to be delivered from the needle intermittently, resulting in incomplete coverage of the web. Clearly, a spore strip which is free, or relatively free of microorganisms will be unable to detect an unsatisfactory sterilization process. When non-absorptive or hydrophobic carriers are inoculated, it is important to be able to tell on which side of the carrier the spores were deposited, for reproducible packaging and subsequent exposure. This recognition is often extremely important for the arrangement of inoculated non-penetrable carriers otherwise the access of a sterilization agent may be hindered.
At present, the method used to evaluate coverage of the paper web is visual observation. Because the microorganism dispersion is generally milky white or clear in color and the blotter paper is also white, it is difficult to determine whether the paper web has been impregnated. Accordingly, an observer constantly monitors the impregnation process to reject regions which are not wetted by the suspension. This method tends to result in a unreliable evaluation of the coverage of the paper web. An observer may fail to spot an unsatisfactory region on the quickly passing paper web. The observer can also monitor the supply of suspension to be sure it is dropping. However, with multi-nozzle systems the suspension still drops when one of the nozzles is clogged.
Additionally, the paper web is not cut to size until after it is dry. Disks for self-contained indicators span a very short length of the paper, typically about 5 mm. At this stage, there is no means of readily checking whether the selected portions of the paper web are coated with microorganisms. As a result, the selected portions may not contain a calibrated microorganism population.
The present invention provides a new and improved indication system for ensuring that a microorganism carrier is impregnated with a population of microorganisms, which overcomes the above-referenced problems and others.